Any discussion of the background art throughout the specification should in no way be considered as an admission that such background art is prior art, nor that such background art is widely known or forms part of the common general knowledge in the field.
Crystalline Raman lasers are efficient converters of pump lasers to longer wavelengths and higher beam quality. The Group IV crystal diamond, which can now be synthesized with excellent optical quality, is especially interesting and has recently been shown to be an outstanding optically-pumped Raman laser material with efficiency, wavelength range, and power exceeding all other materials owing to its high thermal conductivity, high Raman gain, and broad optical transmission range. By all of these measures, diamond is outstanding among all other known materials and has the potential to enable miniature Raman lasers of unprecedented average power and wavelength range. The recent availability of high optical quality synthetic diamond crystals grown by chemical vapour deposition (CVD) is currently enabling a surge of interest in diamond Raman laser development.
Much like the electronic industry, lasers are being developed with ever increasing power, speed and frequency range. Almost all fields of science and technology now benefit from laser technology in some way and demand a range of specifications that will include output wavelength, beam power, temporal format, coherence and system parameters such as footprint and efficiency. Thus there is an ongoing search for alternatives to the optical gain material that is fundamental to laser performance. Diamond is highly attractive as a laser material as it promises capabilities well beyond that possible from other materials in accordance with its extreme properties.
Most diamond laser research to date has concentrated on doped diamond for color center lasers, semiconductor diode lasers and rare earth doped lasers. Success has been very limited except from perhaps color center lasers relying on the nitrogen vacancy that have been demonstrated with an optical-to-optical conversion efficiency of 13.5% [see S. C. Rand and L. G. DeShazer, Opt. Lett. 10, 481 (1985)]. The major challenge for diamond as a laser host is the incorporation of suitable concentrations of color centers or active laser ions into the tightly bonded lattice either by substitution or interstitially. On the other hand, Raman lasers rely on stimulated scattering from fundamental lattice vibrations and thus do not require doping. Though the principle of optical amplification is distinct from conventional lasers that rely on a population inversion, in many ways Raman lasers have similar basic properties to other laser-pumped lasers. Raman lasers can be thought functionally as laser converters that bring about a frequency downshift and improved beam quality. Their development has been most often driven by the need for laser wavelengths that are not fulfilled by conventional laser media and find use in a diverse range of fields such as in telecommunications, medicine, bio-diagnostics, defence and remote sensing.
Synthetic (CVD) single crystal diamond has become available in the last few years with size, optical quality and reproducibility well suited for implementation in Raman lasers. Diamond's starkly different optical and thermal properties compared to “conventional” materials are of substantial interest for extending Raman laser capabilities. Diamond has the highest Raman gain coefficient of all known materials (approximately 1.5 times higher than barium nitrate) and outstanding thermal conductivity (more than two orders of magnitude higher than most other Raman crystals) and optical transmission range (from 0.230 μm and extending to beyond 100 μm, with the exception of the 3-6 μm range due multiphonon interactions). Most solid state Raman materials are only transmissive at wavelengths less than 4 micrometers (silicon being one of the only exceptions).
The potential for diamond to generate radiation in the mid-infrared, long wave infrared, far-infrared and terahertz is of major interest for many applications and may address a serious lack of powerful and practical laser sources at wavelengths between 6 and 100 μm. The wavelength range is in a notorious gap between current optical and electronic microwave sources, but is a rich arena for applications and research in physics, biology, material science, chemistry and medicine including several that are of major significance such as remote and stand-off sensing of bioagents, contraband and toxic chemicals, industrial process monitoring and control, environmental monitoring and biological ‘lab-on-a-chip’ devices. This wavelength region is vitally important for sensing, probing and interacting with our environment and encompasses the molecular “fingerprint” region at one end (5 to 20 μm) to “T-rays” (50 to 200 μm) that safely penetrate many organic materials.
For example, lasers are commonly used in surgical procedures as they offer good precision, the option for keyhole fibre delivery, and reduced bleeding. A major limitation to the range of indications and efficacy is caused by the low spatial precision with which the laser beam power is deposited into the tissue. For example, neurosurgical procedures like the excising of brain tumours cannot often be carried out with current laser technology as the beam power is not deposited in the cells directly but rather chromophores that surround the cells such as water and melanin. The wavelength 6.45 μm has been identified, however, as a key absorption wavelength for providing strong absorption by the amide-II band of proteins and relatively low absorption in water. Lasers at 6.45 μm potentially offer surgeons the capability to ablate tissue with resolution at the single cell level (<5 μm) and a new option to treat otherwise difficult indications. Proof of principle studies undertaken with a free-electron laser at Vanderbilt University USA [see Edwards, G. S., Nature 371, p 416 (1994)] demonstrated efficient ablation and very low collateral damage, and the system was subsequently used in successful human brain and ophthalmic surgical trials [see for example Koos, K. et al., Lasers Surg. Med. 27, p 191 (2000)]. Free electron lasers are, however, large scale (building-sized), costly and inefficient installations only suited to small trials. More practical alternatives have been investigated, but the size and performance requirements for widespread use has yet to be met. The major hurdle to be overcome is that, to date, no solid state laser material has been identified as being capable of generating the required wavelengths and power levels for efficient operation.
The extension of the operation of solid-state, laser-based optical parametric oscillators has been considered using nonlinear materials such as ZnGeP, AgGaSe2, and GaAs, but at present surface damage by the pump laser pulse is an unsolved problem and wavelengths are limited to less than approximately 20 μm. Though quantum cascade semiconductor diode lasers are very promising devices, there are several severe limitations that have impeded their widespread acceptance; peak and average output powers are low (<100 mW), the tuning range is narrow, and cryogenic cooling is often required. The only source that offers wide tunability and high power are multi-million dollar large-scale installations based on high energy electron accelerators (eg., free electron lasers and synchrotrons), which are irrelevant to most practical applications. As a result, the development of practical tabletop or smaller sources as proposed here stands to make a major impact.
Although diamond has long been known to be an interesting Raman laser material, it has only been the last few years in which Raman lasers have been demonstrated. In fact, not long after the discovery of the Raman effect by Raman and Krishnan in 1928, Ramaswamy discovered the strong and isolated 1332 cm−1 Raman mode in diamond [see C. Ramaswamy, Indian J. Phys. 5, 97 (1930)]. Diamond was one of the first crystals that were used to exhibit SRS [see G. Eckhardt, D. P. Bortfeld, and M. Geller, Appl. Phys. Lett. 3, 137, (1963)]. Though in principle Raman lasers made can be from natural diamond, indeed resonant effects in an uncoated natural diamond crystal were observed in 1970 substantial diamond Raman laser development has been limited due primarily to the lack of a reproducible supply of optical quality material provided by synthetic growth methods, which is only recently becoming available.
An important technical challenge results from the two- and three-phonon band in diamond (>0.5 cm−1) which absorbs strongly in the range 3-6 μm. For pump wavelengths longer than 3.8 μm, it is important to consider strong absorption of the pump. Absorption of the first Stokes wavelength is also a consideration for pump wavelengths shorter than 3.2 μm. A further challenge for generating long wavelengths is the diminishing gain that normally occurs when Raman scattering longer wavelengths.
It is an object of the present invention to substantially overcome or at least ameliorate one or more of the disadvantages of the prior art, or at least to provide a useful alternative.